1. Field of the Invention
The field of the invention relates to computed tomography (“CT”)-based threat detection systems generally, and more particularly to certain new and useful advances in using X-ray Diffraction (“XRD”) to resolve false alarms generated by a megavoltage CT threat detection system, of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same.
2. Discussion of Related Art
Megavoltage CT has been developed for inspection and screening of shipping containers, as it provides a high-resolution data 3-D data set of the density and approximate atomic number distribution inside the container. That said, Megavoltage CT is not material-specific, and has been found to generate a significant number of false alarms for some cargo categories. Some of these alarms can be cleared employing On-Screen Alarm (“OSAR”) protocols. The remaining alarms, however, need to be cleared by a secondary technique or failing that, in a worst-case scenario, containers that generate an alarm must be manually inspected. It is desirable to avoid employing a secondary technique or manually inspecting an alarm as both operations are extremely costly.
XRD is a material-specific analysis technique that permits the local diffraction properties of a selected volume element (voxel) of an extended object (e.g. a suitcase, a shipping container, and the like) to be determined. The idea of operating an XRD system as a second inspection modality following a first CT investigation as a way of reducing the false alarm rate in airport baggage screening has been generically outlined in the scientific literature, but the mere mention of this idea left many problems unsolved.
One such problem is whether and how high density metals (“HDMS”) and/or shielded special nuclear materials (“SNMs”) present in shipping containers, passenger baggage, and the like can be detected using a CT-based threat detection system in combination with an XRD-based threat detection system. Another problem is how to optimize an XRD threat detection system to decrease its total X-ray attenuation. Another problem is how to permit simultaneous XRD imaging from a one-dimensional array of object voxels.
Accordingly, an improved threat detection system is needed that detects shielded SNMs, decreases or eliminates false alarms, and that determines a minimum attenuation path through an object, such as a shipping container, a piece of baggage, and the like. Additionally, an improved secondary collimator is needed that enables simultaneous XRD imaging from a one-dimensional array of object voxels.